Globally feral omnivorous animals cause remarkable environmental, biodiversity, agricultural and or industry damage. For example, omnivorous rodents are a global problem requiring vast resources to control. In New Zealand, introduced brush tail possums now number an estimated 60-70 million. The damage they cause is both environmental and financial: they prey on eggs and chicks of native birds and on native insects, damage native forests and spread bovine tuberculosis, posing an immense threat to dairy and beef industries.
In Australia, feral pigs are estimated to number in excess of 4 million with some estimates as high as 25 million. Feral pigs occupy some 40% of the land mass of Australia. These population estimates mean that there may be more feral pigs in Australia than grazed cattle. Feral pigs inhabit, and are well adapted to, a wide range of habitats that include sub-alpine, temperate, sub-tropical, tropical and arid zones, and they are present in most Australian states and territories. While brush-tail possum over browsing of native and agriculturally important flora in Tasmania is also of some concern.
Furthermore, in the United States, the presence of some 4 million feral pigs (also referred to as hogs, boar, or swine) has been reported in some 28 states ranging from California to Virginia, the majority residing in Texas. Feral pigs are the most abundant introduced ungulate in the United States.
Feral pigs have a very high fecundity and frequently raise litters in excess of 6 piglets per breeding sow. Thus, the capacity for feral pig populations to respond to control measures or to totally exploit a food supply is large.
Feral pigs adversely impact agricultural production, environments and ecosystems. A number of studies have identified a range of environmental and agricultural problems arising from feral pig infestations (Alexiou (1984) Effects of feral pigs (Sus scrofa) on sub-alpine vegetation at Smokers Gap, ACT, Proceedings of Ecological Society of Australia, 12: 135-142; Tisdell, C. A., (1982) Wild Pigs: Environmental Pest or Economic Resource? Pergamon Press, Sydney; Miller, B. and Mullette, K., (1985) Rehabilitation of an endangered Australian bird: the Lord Howe Island Woodhen, Tricholimnas sybvestris, Biological Conservation, 34: 55-95; Mitchell, J. and Mayer, R., (1997) Digging by feral pigs in the wet tropics world heritage area of north Queensland, Wildlife Research, 24: 591-601; Choquenot, D., McIlroy, J. and Korn, T., (1996) Managing Vertebrate Pests: Feral Pigs (Ed. M. Bomford) Bureau of Resource Sciences, Australian Government Publishing Service, Canberra 163 pp; Mitchell, J., (2000) Ecology and management of feral pigs in tropical rainforest, Unpublished PhD Thesis, James Cook University of North Queensland, Townsville; Hone, J., (2002) Feral pigs in Namadgi National Park: dynamics, impacts and management, Biological Conservation 105: 231-242); Singer, F. J., Swank, W. T., and Clebsch, E. E. C., Effects of wild pig rooting in a deciduous forest., Wildlife Management 48: 464-473; Lacki, M. J., and Lancin, R. A., (1986) Effects of wild pigs on beech growth in Great Smoky Mountains National Park, Journal of Wildlife Management 50: 655-659. The key points from these various studies are summarised below.
The predatory behaviour of feral pigs causes major economic damage for animal production enterprises over wide areas. The damage is so severe that some areas cannot sustain productive grazing due solely to the widespread infestation of feral pigs. The species' impact on agricultural production has been conservatively estimated to be in excess of 100 million Australian dollars annually.
Feral pigs also cause significant damage to the environment due to their destructive foraging habits which include digging (rooting) and consumption of plants. This destructive behaviour can damage infrastructure including fencing, dams and levy banks and also causes damage to wide areas of fragile riparian habitat. Their effect on native animal species is unknown but is likely to be severe in view of their predatory behaviour and competition for food resources.
Apart from direct damage to grazing enterprises and the environment, the biggest risk from feral pigs arise from their capacity to harbour several major human and animal diseases. Many diseases are zoonotic and the pig provides an ideal amplifying host. Japanese encephalitis virus, leptospirosis, brucellosis and melioidosis have already been detected in feral pigs in Australia. However an even greater risk comes if there is an incursion of foot and mouth disease (FMD) virus into the feral pig population, where the cloven hoofed pigs provide an amplifying host and virus carrier that is widely distributed and highly mobile.
In the United States, pseudorabies virus (PRV) has been eradicated from domestic pigs however PRV continues to circulate in feral pig and raccoon populations. Accordingly, feral omnivore populations can also be a reservoir for fresh PRV outbreaks.
Details of the environmental, human health, animal health and agricultural production problems that arise already, or which might arise, from an unchecked expansion in feral pig numbers are provided in the book “Managing Vertebrate Pests: Feral Pigs” (Choquenot, D., McIlroy, J. and Korn, T., (1996) Managing Vertebrate Pests: Feral Pigs (Ed. M. Bomford) Bureau of Resource Sciences, Australian Government Publishing Service, Canberra 163 pp. Infestation of other omnivorous species such as raccoons, collared peccaries, opossums, possums and rodents can give rise to similar adverse agricultural, environmental, financial and health concerns in various countries.
There is therefore a considerable effort focussed on a reduction of the risks posed by feral omnivorous species in Australia, United States, New Zealand and other parts of the world which have unchecked populations of such species.
Despite their impact, the control of omnivores such as feral pigs, possums, raccoons, and rodents is generally time-consuming, ad-hoc and reactive rather than pro-active management. Many techniques are currently employed for mainly localized control (eg shooting, trapping, fencing, etc) however it has been recognized that broad-scale and integrated baiting campaigns are most cost-effective for reducing and maintaining feral omnivore populations at low levels across large areas. Typical baiting campaigns include ground baiting and aerial baiting where the bait is dropped from an aircraft into the loci of the target omnivore population to be controlled.
Lethal baiting campaigns include the use of various poisons, for instance, sodium fluoroacetate (1080) which is placed in or deposited on cereal grains, fermented grain, compressed bran/pollard pellet baits, fresh or dried meat, offal, carcasses, lupin seeds, and fruit and vegetables, and also cyanide in manufactured baits. Of these, the use of soaked or dry wheat grain or fresh meat baits are the most common.
Of the various control means discussed above poison baiting of feral pig and other omnivore populations is recognised as one of the most effective means of controlling such populations and reducing the damage they cause. Unfortunately however, one of the main problems with many of the bait types made from grains or meat and carcass offal or pellets is that they exhibit poor target specificity. Accordingly, while the commonly employed baiting campaigns may prove effective in controlling feral omnivore (eg pig) numbers in a particular area, such campaigns may also adversely affect individuals of other species of animals which may be desired or native species of animals or birds which come into contact with the baits.
Other disadvantages of the present baiting regime can be attributed directly to the specific poison used. For instance, a disadvantage of 1080 is that feral pigs appear to be relatively resistant to the effects of the poison compared to rabbits, foxes, and wild dogs for which it is a more ideal poison. For example, during captive trials with bait delivered 1080, (McIlroy et al, Australian Wildlife Research 16: 195-202) dingos required 0.11 mg/kg to receive an LD50 dose whereas feral pigs were reported to require at least 1 mg/kg and some as high as up to 4.11 mg/kg (O'Brien et al, Australian Wildlife Research 15: 285-291).
Moreover, while the terminal toxic events associated with 1080 toxicosis are not thought to be accompanied by conscious pain, there are disturbances in the central nervous system and thus effects on behaviour that can appear unpleasant to the untrained observer. Humans that have recovered from nearly lethal exposures to 1080 have not recalled pain after the event however the final phases of toxicosis have been likened to hypoglycaemic or epileptic fitting. Sodium fluoracetate is presently one of the best toxin choices for feral pig management. However the high doses required for feral pigs mean that this is not a perfectly suitable toxin for pig management.
People poisoned with other poisons such as phosphorus (CSSP) or strychnine have reported substantial pain and suffering and it is highly likely that such poisons are too inhumane to be used to control feral animals such as the pig. Similarly, while warfarin is used therapeutically in low doses for humans suffering from blood clotting disorders and this use is not associated with pain, the use of this anticoagulant in large animals such as feral pigs may give rise to painful haemorrhaging in some animals and therefore it is also not a preferred poison for this application.
This would indicate that none of these poisons and conventionally used food based baits, are perfectly suitable or represent perfectly humane alternatives for eradicating or controlling pest species.
The present invention serves to address at least some of these shortcomings.